Mercury — also known as quicksilver — is a naturally occurring silvery, liquid metal released from rocks, soil and volcanoes. It is the only metal that is liquid at room temperature and normal pressure.

While mercury is now widely understood to be a neurotoxin that can adversely affect children's brain development and adult neurological health, it has been used for centuries in various industrial processes and consumer products.

Citing health concerns, Canada stopped the production of mercury metal in 1975. In February 2011, Ottawa also proposed a ban on the manufacture, import and sale of most mercury-containing products starting in 2012.

The proposal would outlaw mercury in thermometers, thermostats and batteries, which, the government says, have comparably priced, technically equivalent non-mercury-containing alternatives.

It would also enforce limits on the amount of mercury allowed in fluorescent lamps, mercury-vapour lamps and automobile headlamps. Other products, however, including dental amalgams and scientific instruments, would not have mercury-content limits.

Such modern-day moves — including a U.S. ban on mercury in latex paints and a European ban against its use in home thermometers — come after centuries of unwitting exposure to its toxic effects.

Quicksilver once prescribed for syphilis

Archeologists have found mercury in tombs dating back to ancient Egyptian and Roman times when people believed it could prolong life and maintain general good health. It was even prescribed as a cure for syphilis.

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The government has proposed a ban on the manufacture, import and sale of mercury-containing products thermometers, thermostats and batteries that would take effect in 2012. ((iStock))

Beginning in the 17th century, hat makers used mercury to process animal furs into felt, breathing in its fumes in poorly ventilated workshops.

The phrase "mad as a hatter" is thought to have originated in the odd neurological symptoms milliners exhibited, including trembling, loss of co-ordination and slurred speech. Even so, mercury was used in the hat-making trade up until the early 1900s.

For a long time, it was also regarded as a curiosity and used in lucky charms, toys and reflecting pools.

In the past century or so, human activity has increased the amount of mercury in the environment in several ways, including through coal-fired power generation, metal mining and smelting and waste incineration.

The metal also plays a big role in many industrial processes — from chlorine production to small-scale gold refinement and atomic clocks. Compounds containing small amounts of mercury are also used as a preservative in some medical preparations, such as vaccines.

Today, Health Canada cautions people to limit their exposure to mercury. Its effect on human health depends on which of its three forms you are exposed to and whether you inhale, ingest it or absorb it through your skin.

Mercury's three forms

Elemental mercury is the form with which most people are familiar and is most dangerous when inhaled. It is a silvery, volatile liquid that gives off a colourless, odourless vapour at room temperature. It's this type of mercury that is found in thermometers, thermostats and barometers.

Accidentally swallowing elemental mercury is not a huge problem because you actually absorb very little mercury this way, says Health Canada. However, breathing it in is another matter because your body readily absorbs its vapours.

Children are especially vulnerable because mercury vapours are heavier than air and pool near the floor where they crawl and play.

At high concentrations, elemental mercury vapour can damage the mouth, respiratory tract and lungs, and can lead to death from respiratory failure.

Dental amalgam fillings contain a mixture of elemental mercury and other metals such as silver, tin, copper and zinc. According to Health Canada, fillings are the largest single source of mercury exposure for average Canadians.

Still, the agency says levels released into the body in this way are not enough to cause illness, and it does not recommend their removal for people who do not show signs of adverse effects.

However, it notes, since mercury can cross the placental barrier and impair kidney function, pregnant women and people with kidney disease should avoid amalgam fillings. That's why many dentists now use a white plastic resin filling instead of amalgam fillings.

The second form of mercury is inorganic mercury, which is formed when elemental mercury combines with other elements such as sulphur, chlorine or oxygen to create compounds known as mercury salts.

The salts can cause kidney failure and gastrointestinal damage. They are also irritating and can cause blisters and ulcers on the lips and tongue, as well as rashes, excessive sweating, irritability, muscle twitching, weakness and high blood pressure, says Health Canada.

Methyl mercury the most dangerous form

The last form is called organic mercury, or methyl mercury, and it is considered the most toxic. It is created when elemental mercury combines with carbon. Some types of bacteria and fungi can change elemental mercury into methyl mercury.

Methyl mercury tends to accumulate to some degree in all fish but especially in predatory fish, because it accumulates in their bodies as they eat other mercury-containing fish.

Methyl mercury is absorbed through the digestive tract, often after the ingestion of contaminated fish. From there, it enters the brain, where it can remain for a long time.

In pregnant women, it can also cross the placenta into the fetus, building up in the fetal brain and other tissues. Methyl mercury can also be passed to babies through breast milk, says Health Canada.

Because children's nervous systems are still developing, they are particularly sensitive to methyl mercury. Effects can include a decrease in IQ, delays in walking and talking, lack of co-ordination, blindness and seizures.

In adults, extreme exposure can lead to personality changes, tremors, changes in vision, deafness, loss of muscle co-ordination and sensation, memory loss, intellectual impairment, and even death.